1. Field of Invention
The present invention relates to an image projecting system. More particularly, the present invention relates to an off-axis image projecting system, which has low fabrication cost and high image quality.
2. Description of Related Art
The image projecting system based on reflection type liquid crystal display (LCD) device usually is categorized into on-axis image projecting system and off-axis projecting system. Wherein, with respect to the on-axis image projecting system, both the illuminating beam and the image-formation beam are traveling along the same path. In this situation, it is necessary to implement a polarization splitter on the path. However, it has several issues for the polarization splitter that the fabrication is rather difficult and expensive, and the projecting system occupies a large space. In comparison with the off-axis image projection system, the illuminating beam and the image-formation beam are traveling along different paths. This can prevent the issues to implement the polarization splitter.
FIG. 1 is a drawing, schematically illustrating a conventional off-axis image projecting system. In FIG. 1, for convenience in descriptions, only the green channel is shown. The off-axis image projection system includes am illuminating source 102, an X plate formed by two color screen plates 150(r) and 150(b), a polarizing plate 104(r, g, b), a field lens 140(r, g, b), a reflective displaying device 108(r, g, b), an analyzer 110(r, g, b), a color cube 160, and a projecting lens set 114. The illuminating source 102 can produce a white light beam, which is split into red, green, and blue components when passing the X plate. The green light is shown in FIG. 1 as the example. The illuminating beam 116(g) travels through the polarizing plate 104(g) and along the light path 118(g) to be incident onto the reflective displaying device 108(g) by a non-vertical incident angle. After the light beam 116(g) is modulated by the reflective displaying device 108(g) and is reflected, an image-formation beam 120(g) is formed. This image-formation beam 120(g) travels along the light path 122(g) through the field lens 140(g), the analyzer 110(g) and color cube 160, and then reaches to the projecting lens set 114 and is reflected. The red and blue light beams (not shown) are also respectively travel through the polarizing plate 104(r, b), the reflective displaying device 108(r, b), the field lens 108(r, b), the analyzer 110(r, b), and the color cube 160, and then reach to the projecting lens set 114 and are reflected. After the image-formation beams 120(r, g, b) in three colors are combined by the color cube 160, they enter the projecting lens set 114.
However, in the foregoing off-axis image projection system, since the light path 118(r, g, b) of the illuminating beam and the light path 122(r, g, b) of the image-formation beam are not at the same path, with respect to the axis direction of the color cube 160 and the projecting lens set 114, the reflective displaying device 108(r, g, b) and the color cubic are not parallel but have an angle. In this manner, the light cone angle for entering the reflective displaying device 108(r, g, b) is relatively large and asymmetric, causing a difficulty to adjust aberration, image clearness, dispersion of pixels, uniformity, and contrast. In order to solve the foregoing problems, when the lens is designed, a projection lens set is added to planar plate 130. In this manner, it increases the difficulty in lens design and the installation. The fabrication cost for forming the lens increases.
In addition, in the foregoing off-axis image projection system, since the reflective displaying device 108(r, g, b) and the color cube 160 are not parallel, and the incident light beam 116(g) travels through the X plate by a tilt angle. The X plate needs to coat the films in two axis directions. However, it is also difficult to fabricate the X plate. The fabrication cost is then high.